1. Field of the Invention
The present invention relates generally to information displays for photographic cameras and, more particularly, to controlling the illumination of such information displays.
2. Description of the Related Art
Many conventional photographic cameras provide information displays to communicate information to the camera user. The information displayed can comprise, for example, the number of exposed frames on the filmstrip loaded in the camera, the present shutter speed and aperture setting, the lens setting, the charging condition of the flash unit, and the like. Typically, such information displays are a type of liquid crystal display (LCD) that are illuminated by power from an energy source, such as a camera battery.
It is advantageous to provide power to the LCD constantly whenever the camera is turned on or is otherwise placed in an active state. If the camera battery is used to power the display, however, operation of camera systems such as the flash unit or the film advance motor can cause flickering of the display. These problems can be somewhat alleviated by adding a large filter capacitor, but this can add size, weight, and expense to the camera. Moreover, supplying the display with power from the camera battery can unacceptably shorten the life of the camera battery. Therefore, cameras that provide constant power to the LCD often include a dedicated LCD battery. Again, however, this adds size, weight, and expense to the camera.
Some cameras include a regulated power supply that is coupled to the camera battery and can constantly supply power to the LCD. Operating the regulated power supply, however, can require a significant current that can quickly drain the camera battery and unacceptably shorten the battery life. An alternative to constantly operating the regulated power supply to illuminate a camera information display is to turn off the power supply or otherwise remove power from the display after a predetermined time interval. Removing display power when the camera is activated, or turned on, is frequently known as providing a "ready" interval or "doze" mode. A user action, such as partly depressing a shutter button, halts the doze mode and restores power to the display. Removing power after a relatively small time interval is not as convenient to the camera user. For increased operating convenience, the predetermined time interval is set to a reasonably extended time, such as one or two hours, to ensure that the display will be illuminated and readily available to the user under most picture-taking situations without the user first pressing the shutter button to illuminate the display. The one to two-hour time interval does not unnecessarily drain the camera battery.
To provide the proper time interval, the camera can include a dedicated timer. The timer is typically started by the completion of an exposure taking sequence. If no additional exposure or other camera operation is performed by the user before the expiration of the time interval, power to the LCD is removed. Thus, the camera battery will not be unnecessarily drained when the camera is not in active use. Such display timers, however, typically require controllers with additional electronic components, such as flip-flops or R-C timing circuits. See, for example, U.S. Pat. No. 4,522,479 to Yamada and U.S. Pat. No. 4,269,496 to Motoori. Such additional components can increase camera cost and complexity.
From the discussion above, it should be apparent that there is a need for a photographic camera information display circuit that removes power from the display after a substantially predetermined time interval has elapsed, without using an additional dedicated display timing circuit or other complicated circuitry or additional components. The present invention satisfies this need.